"Why Don't We Have" is a PopMech series explaining just why some of the technologies promised by science fiction have yet to become fact. Today: the space elevator.

Why fly a rocket into orbit when you can just hop in an elevator? Since it served as the focal point for Arthur C. Clarke's The Fountains of Paradise in 1979, the concept of a space elevator¬ has permeated both sci-fi culture and futurists' plans for space exploration.

Basically, a space elevator would consist of a long cable extending from the surface of the planet about 22,000 miles up into geosynchronous orbit, where the end-point would move in sync with the planet's rotation and stay above the same point on the surface at all times. Compared to using rockets, space elevators would make carrying huge loads of cargo¬ and passengers a breeze. It's a much more feasible plan for leaving the star system and makes space colonization a serious possibility.

Unlike some sci-fi technologies that exist but are wildly expensive or impractical for widespread use, such as the jetpack, the space elevator isn't close to reality. But why?

The biggest problem is finding a material strong enough to serve as the elevator's cable. Carbon nanotubes, the most commonly proposed solution, aren't stable enough when used in such great lengths. Keith Henson, engineer and co-founder of the National Space Society, told io9 in an interview that the hexagonal bonds that make carbon nanotubes so strong would come apart like "a run in a lady's stocking" if laid out for even a fraction of the distance needed. So far, scientists have only produced carbon nanotubes a few inches long and one nanometer wide.

In 2002, when writing a proposal to NASA for a space elevator, Laine and his group predicted that carbon nanotubes would be ready for use in two to three years. Nanotubes had existed for only about a decade at that point; Laine says he now realizes his team didn't know enough about the material to make accurate predictions about how quickly it would advance. But he's still optimistic; today Laine says they were a few decades away at most.

Because of their potential to replace silicon in computer chips, carbon nanotubes have attracted the attention (and funding) of computer companies around the globe. Those companies are investigating the electrical properties of nanotubes—not the mechanical properties that interest Laine—but he says all the attention by large companies will help to solve the issues of mass producing nanotubes.

In the meantime, LiftPort is working on building a moon-based space elevator. "It's an essential stepping stone," Laine says, "and we have all the parts." The lower gravity of the moon, as well as the absence of wind and ice, mean that a space elevator cable doesn't need to be nearly as strong¬. Kevlar is a strong candidate, though it would need some kind of coating to protect it from UV degradation. Laine says a relatively strong man standing on the moon could hold the system in place.

LiftPort wants to put an elevator on the moon by the end of the decade, and an extremely successful Kickstarter campaign over the summer has started them on their way.

As for an earth-based space elevator, it all hinges on carbon nanotube development, and current research prioritizes their use in cell phones over their ability to haul space cargo. But it's not an impossible goal, and there's always the chance of a dark horse overtaking carbon nanotubes as our dream material.